1. Field of the Invention
The present invention relates to a method of radio broadcasting providing continuity of service and a system implementing the method.
Hitherto, narrow-band radio broadcasting systems have used purely analog modulation methods such as amplitude modulation, frequency modulation or single sideband modulation. The last-named modulation is rarely used.
In the near future, radio broadcasting systems known as “digital” systems are going to be standardized. These systems are designed for use in the bands located below the 30 MHz band, and will comply with existing channels, most usually having bandwidths of 9 kHz or 10 kHz.
Although these systems give high-quality, crystal-clear sound, they are handicapped by a phenomenon inevitably linked to digital methods, namely the total disappearance of the audio signal when the reception conditions are poor, for example excessively severe frequency-selective fading or flat fading, a generally inadequate signal-to-noise ratio, interference, etc. This is not the case for amplitude modulation transmission, for example for which, even at very low signal-to-noise ratios, the sound remains intelligible although highly noise-infested and/or distorted, a feature to which the faithful listeners of this type of broadcast are accustomed.
2. Description of the Prior Art
Various advanced methods have been proposed to overcome this problem at least partially.
One of these methods is the UEP (unequal error protection) method in which the bit stream is divided into two parts as follows:                a specially protected part, which may be received with a lower signal-to-ratio than the remainder and conveys a bit stream that is sufficient to regenerate lower quality audio conditions but is deemed to be sufficient to provide minimum continuity of service,        an unprotected part which, when it is properly received, gives the quality of the frequency audio signal referred to here above.        
However, a method of this kind always has the above-mentioned drawback because, when the signal-to-noise ratio is too low, the audio signal is quite simply cut off.
An object of the present invention relates to a method and system of radio broadcasting adapted to ensuring total or almost total continuity of reception whatever the conditions.
A system of this kind is called a permanent audio link (PAL).